Moddable Games

Okay, so I was sitting in class thinking that I need to start some kind of hobby project, and I started thinking about getting some game to make a mod for, albeit very slowly, but eventually. So, I was wondering if there was a preference to which game might be "easier" to mod (more easily understood that is), or if anyone has some pointers, they would be very appreciated. As of now, I am leaning towards getting Neverwinter Nights 2, but maybe someone could give me a diferent direction. Cheers!